Electric motors are capable of very high workloads, but a tradeoff exists between torque output, electric current usage, electric motor speed and working voltage. To increase the torque output of typical electric motors, more current is required, thereby necessitating larger batteries and control equipment. In some applications, such as light electric vehicles, batteries having sufficient capacity to generate torque outputs desirable for propulsion may have a size, weight and/or cost that is disadvantageous or impractical.
It is common to use a speed reducer with electric motors to produce a more usable work output. Speed reducers multiply the input torque of the electric motor by a factor equal to the reduction ratio. A large reduction ratio reduces the amount of electrical current the motor requires to meet a desired level of torque for the application. Conventional speed reducers utilizing planetary, pinion or conventional chain reductions are typically limited to relatively low reduction ratios. For light electric vehicles, multiple stages of conventional speed reducers may be required to achieve adequate performance. However, such multi-stage reducers consume precious space, while adding undesirable weight, complexity and cost to the vehicle.
One example of a light vehicle that may be amenable to electric propulsion is a bicycle. The addition of an electric motor to assist or replace human effort on a bicycle is a relatively new field. In-wheel (hub) motors are the oldest and arguably least desirable approach, while newer systems power the pedal crankset and utilize a freewheel to disengage the pedals under electric assist. Powering the existing bicycle crankset may be desirable in that it allows the motor to use the appropriate gear for the situation as selected by the user or other system. However, a bicycle crankset is geared for human use, and is therefore geared for a relatively slow rotational speed (typically ˜100 revolutions per minute). In general, electric motor efficiency increases and size decreases as the rotational speed of the motor increases, with optimal motor rotational speeds far exceeding the range in which a human can pedal. Currently available methods of electrically powering a cycle utilize common reduction strategies such as multiple stage belt/chain setups, planetary and pinion/spur gear arrangements. Single stage reduction ratios are typically 2:1 to 10:1 and cumbersome to adapt to the open nature of a cycle. With weight and size considerations being particularly critical to electric bicycle and cycle applications, overall reduction ratios are practically limited to 30:1 by combining multiple conventional stages.